The present invention relates to an apparatus for electric discharge machining which is capable of effectively cutting and shaping a workpiece in a desired configuration with extremely high accuracy.
FIG. 1 is a schematic circuit diagram showing an example of a conventional apparatus for cutting and shaping a workpiece using an electric discharge with a wire-shaped electrode by the use of a voltage-operated servo-control mechanism. The conventional apparatus, as shown in FIG. 1, includes a wire-shaped electrode 1, a workpiece 2 to be cut and shaped into a desired configuration, an electric supply contact 3, an electric power source 4, an error voltage amplifier 5, an operational amplifier 6, resistors 7 and 8 for determining the gain of the system, a drive control device 9, an X-axis drive motor 10, and a Y-axis drive motor 11.
A pulsating current supplied from the electric source 4 is applied through the electric supply contact 3 to the wire-shaped electrode 1 to thereby cause an electrical discharge which is used to machine the workpiece 2. A gap voltage E.sub.g applied between the wire-shaped electrode 1 and the workpiece 2 during the machining operation is applied to the error voltage amplifier 5 where the gap voltage E.sub.g is then compared with a reference voltage E.sub.o to produce an error voltage -E.sub.e representative of a difference therebetween. In this case, -E.sub.e =-(E.sub.g -E.sub.o).
The error voltage -E.sub.e is applied to the operational amplifier 6 where the polarity of the error voltage -E.sub.e is inverted and the resulting signal is applied as a table speed signal F. Reference numerals 7 and 8 designate resistors for determining the gain of the system, the gain being determined by the ratio of the resistance values R.sub.1 and R.sub.2, specifically R.sub.2 /R.sub.1. The drive control device 9 operates to divide the table speed signal F into an X-axis component F.sub.x and a Y-axis component F.sub.y where F=F.sub.x.sup.2 +F.sub.y.sup.2. The values of F.sub.x and F.sub.y are determined according to the contents of an N/C memory tape. The signals F.sub.x and F.sub.y are coupled respectively to control the X-axis drive "motor" 10 and the Y-axis drive motor. In such a voltage-operated servo-control mechanism, if the discharge gap between the wire-shaped electrode 1 and the workpiece 2 is increased, the gap voltage E.sub.g increases resulting in an increase in the error voltage E.sub.e . As a result, the table speed signal F increases and the table supporting the workpiece 2 moves in a direction so as to reduce the length of the discharge gap.
On the other hand, if the discharge gap is reduced, the table speed signal F is reduced. Consequently, the table moves in a direction so as to enlarge the discharge gap. That is, with this system, it is possible to maintain the length of the discharge gap constant by maintaining the gap voltage constant during machining. Since the length of the machining gap is always maintained constant, machining of the workpiece with an accuracy higher than that attainable with a system in which the table is always driven at a constant speed is possible.
The operation of the conventional apparatus for driving the electrode employing a voltage servo-control mechanism has been described. In this system, since the table speed F is determined by the error voltage E.sub.e and the gain R.sub.2 /R.sub.1 of the operational amplifier, it is impossible to make the error voltage E.sub.e equal to zero (E.sub.e =0). In addition to this fact, while the table speed F during the machining can be expressed by: EQU F=(R.sub.2 /R.sub.1).multidot.E.sub.e, (1)
due to variations in wire tension, variations in specific resistance of the machining liquid which unavoidably occur during the machining operation, directional characteristics of the wire guide and the like, even through the gap voltage during the machining is maintained constant, the table's speed may change slightly. Assuming that the amplitude variation of the table speed is .DELTA.F, from equation (1), .DELTA.F can be expressed as: EQU .DELTA.F=(R.sub.2 /R.sub.1).multidot..DELTA.E.sub.e .multidot.(2)
Accordingly, the error voltage E.sub.e also varies by .DELTA.E.sub.e. As described hereinbefore, E.sub.e =E.sub.g -E.sub.o and the reference voltage E.sub.o is constant. Therefore, .DELTA.E.sub.e =.DELTA.E.sub.g. From equation (2), EQU .DELTA.E.sub.e =.DELTA.F/(R.sub.2 /R.sub.1).multidot.
Thus, EQU .DELTA.E.sub.g =.DELTA.F/(R.sub.2 /R.sub.1).multidot. (3)
In other words, in the conventional electric discharge machining apparatus with a wire-shaped electrode employing a voltage-operated servo-control mechanism, in order to maintain the gap voltage constant, the table speed is varied. However, if this speed varies .DELTA.F, as indicated by equation (3), the gap voltage varies by (R.sub.2 /R.sub.1).DELTA.F. The variation in the gap voltage results directly in variations in the gap length. This results in a disadvantage that machining with a high accuracy is impossible. From equation (3), it may be seen that, in order to minimize the variation in the gap voltage with respect to that of the table speed, the ratio of R.sub.2 and R.sub.1 should be increased. However, since the ratio R.sub.2 /R.sub.1 determines the gain of the voltage-operated servo-control mechanism system, if the gain is increased, the system may be entirely put in an unstable state or the table speed may greatly vary for only a slight disturbance. In view of the above, in the conventional apparatus, the operational amplifier is provided with phase compensating means to increase the real gain R.sub.2 /R.sub.1 whereby the gap voltage during machining is made as constant as possible. Nonetheless, the conventional apparatus is disadvantageous in that the gain cannot be made infinite and the gap voltage during machining is liable to change thereby resulting in degradation in the machining accuracy.